This invention relates generally to the field of X-ray detector assemblies for medical imaging and more particularly to the construction of X-ray detector assemblies with dual para-xylylene layers.
In an X-ray detector assembly, an amorphous silicon detector substrate is coated with a vapor phase deposited X-ray scintillator material. The scintillator material generates photons isotropically from the absorption of the X-rays. A reflective layer is required to reflect photons, which are emitted in a direction away from the detector substrate, back towards the detector substrate. A typical reflective layer (such as an Opticlad™ film, a registered trademark product available from the General Electric Company, Pittsfield, Mass.) covers the scintillator material. A detector matrix array subsequently measures the intensity of these photons. A moisture resistant seal is disposed between a moisture resistant cover and the detector substrate near the periphery of the X-ray detector assembly.
One important factor in medical imaging applications is in the detector spatial resolution. Photons, which are generated in the scintillator material over one detector pixel, must be counted only by that underlying pixel to obtain a high image resolution. Photons that are scattered to adjacent pixels reduce the clarity of the image. To this end, the scintillator material is vapor deposited in columnar or needle form. Individual needles are separated from one another and they possess aspect ratios (length/diameter) of 100 or greater. Photons traveling down the scintillator needles tend to be contained within the individual needle due to the higher refractive index of scintillator material over air, provided that the individual scintillator needles remain separated. The Cesium Iodide (CsI) scintillator material is known to be a very hydroscopic salt. Exposure of the CsI scintillator material to moisture can cause the CsI scintillator material to absorb the moisture, which further causes the individual CsI scintillator needles to fuse together.
One source of moisture that could effect the CsI scintillator material is the moisture that is contained in the pressure sensitive adhesive (PSA) layer of the Opticlad™ film that is used to attach the Opticlad™ film to the scintillator material. This Opticlad™ film reflective layer is placed over, and is in direct contact with, the CsI scintillator material.
However, applying the reflective layer reduces the detector image spatial resolution after the X-ray detector assembly is heated for several months at a temperature range between about 30 and about 35 degrees C. (e.g., conditions approximating normal operating environment). The MTF (Modulation Transfer Function) of the X-ray detector assembly is reduced, by a value of about 20% or greater, as a result of the moisture contained with in the PSA layer of the Opticlad™ film. The MTF is defined as the modulation of the image divided by the modulation of the object. Where:
  Modulation  =            (                        maximum          ⁢                                          ⁢          luminance                -                  minimum          ⁢                                          ⁢          luminance                    )                      (                              maximum            ⁢                                                                      ⁢                                                                    ⁢            luminance                    +                      minimum            ⁢                                                  ⁢            luminance                          )            ⁢                          
A second source of moisture is ambient environment moisture diffusion through the moisture resistant seal that bonds the moisture resistant cover to the detector substrate. This ambient environment moisture can degrade X-ray detector assembly performance. A third source of moisture is the moisture that is absorbed by the CsI scintillator material during X-ray detector assembly fabrication outside of the dry vacuum environment before sealing the detector substrate to the-moisture resistant cover. Unless the X-ray detector assembly is sealed in a controlled, very low relative humidity ambient, the CsI scintillator material is exposed to moisture vapor during the assembly process during application of the moisture resistant seal. The CsI scintillator material has the potential to absorb moisture, which can degrade the performance of the X-ray detector assembly.
It is desirable to have an X-ray detector assembly design that minimizes the amount of moisture absorbed by the scintillator material from moisture sources inside the X-ray detector assembly. It is further desirable to have a robust seal assembly that protects the scintillator material and the structure holding the scintillator material from penetration by ambient moisture. It is further desirable to physically protect the X-ray detector assembly from damage caused by handling. It is also desirable that any encapsulating coating disposed over the scintillator material be easy to apply at temperatures less than about 250 degrees C. and the encapsulating coating fully contain the high aspect ratio scintillator needles. It is further desirable that any encapsulating coating forms a mold-like structure, reducing the moisture that can get into the CsI scintillator material, and constrain the CsI scintillator needles from touching adjacent scintillator needles to reduce X-ray picture degradation. It is further desirable that the encapsulating coating applies no distortion onto the scintillator material.